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Re: hot-add unformatted drive to RAID array automagically

On Thu, 2005-02-17 at 23:29 +1100, Russell Coker wrote:
> A RAID-5 setup without NVRAM for write-back cache will give poor write 
> performance.  If you want to write one sector to a RAID-5 array then the 
> controller has to read the original sector and the checksum sector and then 
> write to the sector and the new checksum.  This makes a single write become 
> two reads and two writes (one read and re-write on each of two disks).  All 
> good hardware RAID systems have NVRAM write-back caches to allow 
> write-combining on RAID-5 lines.  This gives a massive performance benefit.

But surely writing 4 sectors (in a 4+1 RAID5) is implemented as writing
1 sector to each disk in parallel (4 data sectors, 1 checksum). So if
you were using a block size of 2K (4 x 512Bytes disc sector size), you
would get a 4 x speedup (assuming checksum calculation is negligible).

Donovan Baarda <abo@minkirri.apana.org.au>

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